This invention relates to a device for detecting X-rays, infrared light and ionizing particles. In particular, this invention relates to an integrated VLSI pixel detector having PIN diodes which are reverse biased and collect the charge generated by an incident beam of radiation or ionizing particles.
PIN diodes, which are reverse biased and collect the charge generated by an incident beam of radiation or ionizing particles, can be used in the detection of such radiation and particles. A detection device using such PIN diodes was disclosed, for example, by one of the present inventors in U.S. patent application Ser. No. 198,357, filed May 25, 1988 and in "A Proposed VLSI Pixel Device for Particle Detection" (Nucl. Instr. and Meth. A275, 494 (1989)), which is hereby incorporated by reference. With such PIN diodes arranged in a one-dimensional or two-dimensional array on a chip, there are different ways of connecting them to readout circuitry for detection. One way is to put the readout circuitry on a separate integrated circuit and then to bump-bond that chip to the detector chip containing only the PIN diodes. Another is to integrate all, or at least a large part of the readout circuitry with the diodes on the same chip. Both approaches have advantages and disadvantages. For application to high energy physics, however, the bump-bonding method has a major disadvantage wherein the two chips bonded together become thicker than one chip which integrates both detector and readout circuitry. This causes significantly more scattering and hence adversely affects the quality of the measurement.
The structure proposed in the references cited above is characterized as having the junction of the PIN diodes created by a diffusion at the front side of its wafer and the readout circuitry contained in wells which are of the same conduction type as the substrate and very close to the junction of the PIN diodes where the electric field is high. To divert the field lines from the well to the collection electrode, in order to collect most of the charge generated by the incident radiation, a high bias is necessary for the well. In order to keep this bias within a reasonable limit, the size of the collection electrode must be made quite large with respect to the well. This results in a significant loss of area on the chip and a considerable loss in (spatial) resolution.